Cabbage worms. They're ubiquitous. You can spray an organic pesticide like bT or neem oil, or if you prefer not to spray anything then row covers (netting over the row to keep the moths from laying eggs) are the way to go.
Personally I patrol my garden daily and inspect plants. I squish the eggs and pick off the larva by hand. I find it relaxing and a great way to connect with the plant, which usually tips me off if something else is awry early enough that I can fix it.
If you have too large of a garden/no time for that, consider spraying or row covers.